Three dimensional printed, free-form fabricated, or additive manufactured objects can be produced fairly simply using known methods, including but not limited to, the deposit of particles, such as, but not limited to, plaster or plastics, or layers, such as, but not limited to, paper, plastic or fabrics. However, the resulting products/objects may be porous, “soft” and easily damaged. It is therefore desirable to develop improved methods for strengthening or otherwise protecting such objects, and/or providing quality finishes thereto.
Current approaches to strengthening/protecting such manufactured objects fall into the following generalised categories:
Impregnation of the Manufactured Objects with an Acrylic Resin.
In general, the acrylic resin is a cyanoacrylate (not favored due to its toxicity) and it is absorbed into pores or voids in the object where it rapidly polymerizes, in the presence of water (specifically hydroxide ions), forming long strong chains which join and bond the particles and/or layers together. However, because the presence of moisture causes the cyanoacrylate to set, exposure to moisture in the air can cause containers of cyanoacrylate to rapidly deteriorate and become unusable over time. To prevent this, the cyanoacrylate must be stored in an airtight container with a package of a desiccant such as silica gel.
Impregnation of the Manufactured Objects with Waxes.
Whilst waxes are safer to handle than cyanoacrylates, and can be “re-melted” allowing reprocessing of the object, they “shrink” in the pores and consequently, generally provide an unsatisfactory finish. Furthermore their hydrophobic nature makes the wax impregnated object difficult to finish.
Impregnation of the Manufactured Objects with a Curable Resin.
Curable resins, such as, heat cured or UV cured resins, like cyanoacrylate, form a permanent finish and can't be reworked if the finish is unsatisfactory. They also shrink in the pores and, due to their viscous nature, often only impregnate the outermost surfaces, meaning the cured product lacks good structural integrity; and
Impregnation of the Manufactured Objects with Two-Part Component Resins.
Typically the two components are pre-mixed so that they will “set”, and are used to impregnate the object whilst they are workable. Examples include: polyesters, epoxy resins, and polyacrylates. Disadvantages include the fact they set in a non-reversible manner, suffer from a poor quality finish and mixing results in wastage.
The disadvantages with these alternative impregnating materials and methods are overcome through the use of a thermoplastic polymer which will impregnate the pores and voids in a molten state in a first defined temperature range and set at a second temperature range (lower than said first).
By the use of e.g. pressure, they can be caused to impregnate the pores and voids deeply (and not just the outer surface), resulting in less shrinkage and greater structural integrity providing a better quality of finish. By deeply is meant able to penetrate the pores by greater than 10%, through 20%, 30%, 40%, 50%, 60%, 70%, 80%, and 90%, to 100% impregnation, depending on the thickness of the object.
Furthermore, unlike cyanoacrylate, the current “gold standard”, they are safe to handle and do not suffer from premature spoiling. More significantly still, unlike cyanoacrylate and cured resins, the impregnation with a thermoplastic can be reversed, if a good finish is not achieved first time around.
Partculate mixtures containing absorbent fillers for three dimensional (3d) printing are known, and US2005/0059757 provides a good overview of the general state of the additive manufacturing art. It describes rapid prototype techniques including a selective laser sintering process and a liquid binder 3d printing process, which both use layering techniques to build 3d articles.
The selective laser sintering process makes cross reference to U.S. Pat. No. 4,863,568 and other techniques make cross reference to U.S. Pat. Nos. 5,204,055, 5,902,441 and 6,416,850.
US20050059757 describes treating an “intermediate article” with an “infiltrant” to define a substantially solid “final article” having approximately 20-70% infiltrant by volume. Infiltration is described in detail and the liquid resin infiltrants described include those cured by either heat, UV light and electron beam, mixing (of two part systems), catalysts and moisture (including the use of cyanoacrylates).
Reference is also made to the use of liquid infiltrants which may be solidified by cooling (and drying) and waxes are the main group of compounds identified.
These are applied to an intermediate article by, for example, brushing, spraying, dripping or dipping and then the intermediate article is placed in an oven or otherwise cured.
The document additionally teaches that the powder constituents used in the printing of additive manufactured objects may contain activating fluids, including phase change materials including thermoplastics which will melt and solidify to bind particles together.
Other art identified includes GB2,283,966 which relates to pulp mouldings made by applying pulp around a mesh. Laminated object manufacture (LOM) is used to produce a support for the mesh which support is formed by bonding sheet materials together. However, low bond strength between the laminae can result in the articles having inadequate structural rigidity. Thus, resins may be used to impregnate the LOM generated article which are designed to include holes or discontininuities in the plastic “bonding layers” on adjacent laminae, such that upon impregnation with a resin permeation can be achieved.
DE 19927923 discloses post treating porus sintered polystyrene components by immersion in a warmed infiltration material, preferably a low melting point wax.
US 2003/0186042 discloses a selective laser sintered (SLS) product in which voids are created, this time by drilling, which voids are filled with a filler material which is poured, injected, vacuumed, extruded, deposited or otherwise introduced. A wide range of filler materials are contemplated.
In contrast to what is disclosed in the art, the present invention utilises:                i) low melting crystalline polymers, more particularly, caprolactones with highly defined functionality (determined by ring-opening addition polymerisation) and low viscosity, enabling good penetration and flowability from the object surface allowing reclaim and quality surface finishing; and        ii) by additionally infusing the material under pressure and oscillating between, typically but not essentially, negative and positive pressures, the thermoplastic material is forced into the object.        
Furthermore, the infused object can be tempered and/or annealed in a curing chamber to ensure targeted crystallinity thereby imparting improved characteristics to the infused product.
Other benefits of the method and infusion apparatus will become apparent hereafter.
With regard to infusion apparatus, U.S. Pat. No. 3,384,505 discloses an apparatus used to infuse or impregnate wound glass fibre packages. In fact, at least two independently operated apparatus are required, as a pre-dried object is placed (from an oven) into a pressure vessel, were a vacuum is drawn to remove air trapped in the intertices to facilitate resin pick up when a liquid resin is fed into the partially evacuated immersion vessel, in which the dried object (a wound fibrous material) is submerged in a liquid medium (containing a heat curable thermoset resin) under pressure and then the object is transferred to an oven for curing.
JP 01-254741 discloses an apparatus for producing ceramic paper comprising an immersion tank filed with resin and a drying oven.
In contrast to the above mentioned apparatus, the apparatus of the present invention is able to control all operations sequentially in a single, multi-functional apparatus. It is able to facilitate pre-treatment (e.g. driving off water or volatiles), immersion to impregnate, drainage of excess material by moving the object (through X, Y and Z axis as required), to displace thermoplastics material; remove excess material from the object using a suction and/or blowing tool and then facilitate curing, including tempering or annealing of the thermoplastics, amongst others. Indeed, in some modes of operation a single chamber can be used to perform both immersion and curing under pressure.
Such an apparatus provides the flexibility to treat unique and different objects in contrast to the apparatus of the art which have been designed to process a single article, such as, wound glass fibre packages or sheet paper.
The favoured thermoplastics of the invention are polycapralactones which are, of course, well recognised polyesters, see e.g US 2007/0111037 and U.S. Pat. No. 5,977,203, with a wide range of applications, and are produced by companies such as Perstop.
It is a first object of the present invention is to provide improved methods for infusing three dimensional printed, free form fabricated or additive manufactured objects.
It is a separate and yet further object to identify functional polymers which are safer and have greater versatility for use with said method, and to provide stronger and/or better finished objects infused by said polymers.
It is yet a further and independent object to provide improved apparatus for the automated infusion of three dimensional printed, free form fabricated or additive manufactured objects.